Date: Sun, 14 Jan 2018 01:47:13 +0100 From: Dimitry Andric <dim@FreeBSD.org> To: J David <j.david.lists@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: FreeBSD 11.1 + Clang 4.0 + PHP source = Core dumps Message-ID: <6ED2FBBD-8404-4979-BC4B-559AE5229B3B@FreeBSD.org> In-Reply-To: <CABXB=RQoLTiDc8_vGhiv95pxN02Twfi_5m0Q%2Bc7OCAKBe9Np0w@mail.gmail.com> References: <CABXB=RQoLTiDc8_vGhiv95pxN02Twfi_5m0Q%2Bc7OCAKBe9Np0w@mail.gmail.com>
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On 14 Jan 2018, at 00:24, J David <j.david.lists@gmail.com> wrote:
>
> Starting with Clang 4.0 on FreeBSD 11.1 we are seeing really odd
> behavior and crashes on a version of PHP that we compile in house.
> It’s not clear if this is a compiler bug, or what exactly is
> happening.
>
> For example, this code (from PHP’s intl extension) core dumps:
>
> U_CFUNC TimeZone *timezone_process_timezone_argument(zval *zv_timezone,
> intl_error *outside_error,
> const char *func)
> {
> zval local_zv_tz;
> char *message = NULL;
> TimeZone *timeZone;
>
> if (zv_timezone == NULL || Z_TYPE_P(zv_timezone) == IS_NULL) {
> timelib_tzinfo *tzinfo = get_timezone_info();
> ZVAL_STRING(&local_zv_tz, tzinfo->name);
> zv_timezone = &local_zv_tz;
> } else {
> ZVAL_NULL(&local_zv_tz);
> }
>
> if (Z_TYPE_P(zv_timezone) == IS_OBJECT &&
> instanceof_function(Z_OBJCE_P(zv_timezone), TimeZone_ce_ptr)) {
>
> If zv_timezone is passed in as a NULL pointer, this code core dumps on
> the “if (Z_TYPE_P(zv_timezone)” and gdb says that zv_timezone is NULL.
> But if you look immediately above, if zv_timezone is NULL it is set to
> another value.
>
> If you add a printf of the zv_timezone pointer above the second if
> block, it will show that zv_timezone is no longer NULL (because it was
> just set to &local_zv_tz), and the program will no longer crash.
>
> This crash can also be “fixed” by placing the following line (a memory
> barrier) above the second if statement:
>
> __asm__ volatile(“" : : : "memory");
>
> Although that addresses this one, it seems like there may be a number
> of other similar issues throughout the PHP code base.
>
> This just can’t be right; that should not be necessary. What’s going on?
>
> Is this a bug in clang? Is PHP doing something dodgy? User error on
> our part? We don’t see this behavior compiling the same source with
> clang 3.x / FreeBSD 10.x.
I suspect that you need to add -fno-strict-aliasing to your compilation
flags, if you haven't already. From a cursory look, PHP seems to use a
*lot* of aliasing and type punning.
-Dimitry
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